Exponential behavior of the Ohmic transport in organic films

An exponential dependence of conductance on thickness and temperature was found in the low-voltage, Ohmic regime of copper (CuPc) and cobalt (CoPc) phthalocyanine, sandwiched between palladium and gold electrodes unlike ever claimed in organic materials. To assure cleanliness and integrity of the electrode-phthalocyanine interface, the devices were prepared in situ, using organic molecular beam deposition with a floating shadow mask. The dc transport measurements in a wide temperature and thickness range show that (i) the low-voltage J-V curve is linear, with current increasing sharply at higher voltages, (ii) the low-voltage conductance increases exponentially with temperature, and (iii) it decreases exponentially with film thickness. A comparison with conventional models fails to explain all the data with a single set of parameters. On the other hand, a model outlined here, which incorporates tunneling between localized states with thermally induced overlap, agrees with the data and decouples the contributions to conductance from the electrode-film interface and the bulk of the film.